The present invention relates to an anesthetic and a method for making the anesthetic. More specifically, the present invention relates to an anesthetic containing propofol as its active component for use in veterinary applications.
Anesthetics are useful in surgical procedures to artificially produce unconsciousness or to reduce sensitivity to pain. Anesthetics are typically viewed as primarily applicable to humans. However, anesthetics also may be administered to all types of animals to reduce pain when setting broken bones, performing internal surgery, or otherwise handling the animal.
One common method of veterinary anesthetization is to premedicate the animal with an alpha-2 agonist such as xylazine or detomidine, and then induce anesthesia with ketamine. The ketamine anesthetic may be followed by the administration of a gas anesthetic to maintain anesthesia for the remainder of the procedure. Another common method of veterinary anesthetization is administering a thiobarbiturate mixed with glycerol guaicolate. An anesthesia gas may then be administered to maintain anesthesia for a prolonged surgical procedure.
The primary disadvantage associated with these two methods is that they require access to a gas anesthetic machine. Many surgical procedures take place in remote areas where such a machine is not available. If the inducing agent used to anesthetize is administered throughout the procedure without supplemental gas, recovery is often difficult and could be violent.
Propofol has been used in anesthetic formulations administered to humans and dogs. These propofol formulations contain a phospholipid, such as egg lecithin, which functions as an emulsifying agent. However, phospholipids are good substrates for bacterial growth. Phospholipids are also incompatible with numerous preservatives that are at least somewhat water soluble, such as benzyl alcohol. The addition of such a preservative to a formulation containing phospholipids could destroy the formulation. Without a preservative in the formulation, any excess formulation must be thrown away within a few hours of its first use. Some formulations containing phospholipids also include a chelating or sequestering agent, such as ethylenediaminetetraacetic acid (EDTA). However, EDTA is not truly an antimicrobial substance and, thus, is not as effective as a preservative in preventing microbial growth. Another disadvantage with propofol formulations currently available is that they typically contain relatively small amounts of propofol, generally less than five percent by weight/volume (w/v). Therefore, large quantities of the formulation must be administered to provide the desired anesthetic effect.
To overcome the deficiencies found with conventional anesthetics, an anesthetic formulation containing a preservative and a method for making this anesthetic formulation are needed in the art. In addition, a single anesthetic formulation that can be used to both initially anesthetize an animal and to maintain anesthetization is needed.
It is therefore an object of the present invention to provide an anesthetic formulation that overcomes these disadvantages.
It is another object of the present invention to provide an effective anesthetic formulation that is compatible with an at least somewhat water soluble preservative so that it may be used multiple times before being thrown away.
It is another object of the present invention to provide an anesthetic that is short acting and has a smooth induction to provide the anesthetized animals with an easy recovery.
It is a further object of the present invention to provide an anesthetic that can be used without supplemental gas so that it can be administered at any location.
Another object of the present invention is to provide an anesthetic that is effective in a short amount of time so that it can be used to induce anesthesia in an animal.
A further object of the present invention is to provide an anesthetic that is safely administered for long periods of time so that it can be administered to maintain anesthetization.
It is another object of the present invention to provide a method for making an anesthetic to achieve the foregoing objects.
According to the present invention, the foregoing and other objects are achieved by an anesthetic that includes of a mixture of propofol, a tonicity agent, a substantially phospholipid-free emulsifying agent, a preservative such as benzyl alcohol, and water. The anesthetic is made by combining these components and then filtering the mixture through a sterilizing filter. The anesthetic may be administered parenterally. The anesthetic may be administered to initially anesthetize and/or to maintain anesthetization.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The anesthetic of the present invention includes a mixture of propofol, a tonicity agent, an emulsifying agent, a preservative, and water. Propofol (2,6-diisopropylphenol) is the active ingredient for this formulation and functions as the anesthetic. It is a sedative hypnotic agent, which can be used for both the induction and the maintenance of anesthesia. The anesthetic may be parenterally administered to any animal, including humans. It is especially useful for horses, cats and dogs. Most preferably, it is administered to horses.
The emulsifying agent in this mixture acts as a bridge between the oily propofol and the water so as to emulsify the mixture. The emulsifying agent has properties of a surfactant and a solvent. The emulsifying agent allows the present formulation to be injected into animals. The agent is substantially devoid of phospholipids and is nonionic. Preferably, the emulsifying agent used in the formulation of the present invention does not contain phospholipids. Preferably, the emulsifying agent is polyethoxylated castor oil. Polyethoxylated castor oil is a good emulsifier because it is well-tolerated in animals and because it is able to be administered parenterally. Furthermore, it is a chemically stable substance and, unlike phospholipids, it resists oxidation and microbial degradation. One brand of polyethoxylated castor oil that may be used is T-DET C-40, which may be purchased from Harcros Organics, 5200 Speaker Rd., P.O. Box 2930, Kansas City, Kans. 66106-1095.
The tonicity agent of the present invention maintains a substantially isotonic formulation. It functions to make the formulation compatible with animal tissue. It also helps prevent the hemolysis of red blood cells in the animal. The tonicity agent may include, but is not limited to, sodium chloride, potassium chloride, mannitol, glycerin, dextrose, or dextrose anhydrous. Preferably, the tonicity agent is dextrose anhydrous.
The preservative of the present invention functions as an antibacterial or antimicrobial agent. It is at least somewhat soluble in water. Parabens, phenols, and benzyl alcohol are among the preservatives that can be used in the formulation. More specifically, the preservative used in this formulation may include, but is not limited to, benzalkonium chloride, benzethonium chloride, benzoic acid, benzyl alcohol, butyl paraben, cetylpyridinium chloride, chlorobutanol, chlorocresol, cresol, dehydroacetic acid, ethylparaben, methylparaben, methylparaben sodium, phenol, phenylethyl alcohol, potassium benzoate, potassium sorbate, propylparaben, propylparaben sodium, sodium benzoate, sodium dehydroacetate, sodium propionate, sorbic acid, or thymol. Preferably, the preservative is benzyl alcohol. The preservative should meet the Antimicrobial Preservative Effectiveness (APE) test. This test requires cultures of each of the microorganisms, Aspergillus niger, Staphyloccus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans to be tested in the formulation. The concentration of viable bacteria in the formulation must be reduced to not more than 0.1% of the initial concentrations by the fourteenth day. The concentrations of viable yeasts and molds must remain at or below the initial concentrations during the first 14 days. The concentration of each test microorganism must remain at or below these designated levels during the remainder of the 28-day test period. The preservative used in this formulation is compatible with the emulsifying agent because a substantially phospholipid-free emulsifying agent is used.
Water functions as a solvent in this formulation. The water used in this anesthetic formulation should be suitable for injection under United States Pharmacopeia (USP) standards. These standards provide that water complying with the U.S. Environmental Protection Agency National Primary Drinking Water Regulations or the comparable regulations of the European Union or Japan be purified by distillation or reverse osmosis. Water, which is suitable for injection according to USP standards, contains no added substances.
The anesthetic of the present invention preferably includes the mixture of about 1-30% w/v propofol, about 1-19.8% w/v tonicity agent, about 5-40% w/v emulsifying agent, and about 0.5-2.5% w/v preservative, if benzoic acid, benzyl alcohol, chlorobutanol, chlorocresol, cresol, dehydroacetic acid, phenol, phenylethyl alcohol, potassium benzoate, potassium sorbate, sodium benzoate, sodium dehydroacetate, sodium propionate, sorbic acid, or thymol is used as the preservative, or about 0.01-0.5% w/v preservative, if benzalkonium chloride, benzethonium chloride, butyl paraben, cetylpyridinium chloride, ethylparaben, methylparaben, methylparaben sodium, propylparaben, or propylparaben sodium is used as the preservative. All percentages are by weight/volume (w/v) unless otherwise noted. The emulsifying agent content varies with the propofol content in a proportional relationship. More preferably, the anesthetic of the present invention includes the mixture of about 9-11% propofol, about 4-18% tonicity agent, about 15-25% emulsifying agent and about 1-2% benzyl alcohol. Most preferably, the anesthetic of the present invention includes the mixture of about 10% of propofol, about 5.4% of dextrose anhydrous, about 20% of polyethoxylated castor oil, and about 1.5% of benzyl alcohol per 100 milliliters of solution wherein a sufficient quantity of water for injection is used to make the balance of the solution.
The anesthetic of the present invention is a substantially isotonic solution, having an osmolarity less than 1. It has a pH between about 5.5 and 9.5.
The anesthetic of the present invention is made by combining propofol, a tonicity agent, an emulsifying agent, a preservative, and water to form a mixture. These components are then mixed for an effective period of time. After these components are thoroughly mixed, the mixture is filtered through a sterilizing filter. The order in which components are added is not critical. Preferably, the remaining water is added last so that a specific quantity of anesthetic may be obtained. This process can be scaled to make any desired quantity of the formulation.
One preferred method of making the anesthetic of the present invention includes placing 30-40% of the water in a vessel, agitating the water, and mixing the tonicity agent with the water until a clear solution forms. Next, an emulsifying agent is mixed with the clear solution until a milky solution forms. Propofol is then mixed with the milky solution for an effective period of time. Typically, it is mixed for at least about 10 minutes. Following this, a preservative is mixed into the solution for an effective period of time. Typically, it is mixed for at least about 10 minutes. Agitation is discontinued, and the balance of the water is added to the solution. The anesthetic formulation is then mixed for an effective period of time. Generally, it is mixed for at least about 10 minutes. Next, it is filtered through a sterilizing filter. Preferably, the anesthetic is filtered through a 0.22 micron or smaller absolute sterilizing filter, wherein the filter contributes to the emulsification process. Most preferably, this filter is made of polytetrafluoroethylene. A desirable polytetrafluoroethylene 0.22 micron absolute sterilizing filter may be purchased from Millipore Corporation, 80 Ashby Road, Bedford, Mass. 01730.